Three different proteins recognize a multifunctional determinant that controls replication initiation, fork arrest...

Type I elements regulate the initiation of DNA replication, elongation of replication forks and transcription of the Tetrahymena thermophila rDNA minichromosome. Previous studies identified a 24 kDa protein, ssA-TIBF, which binds the A-rich strand of type I elements. Here we describe two additional type I element binding activities (native mol. wt approximately 65 and approximately 250 kDa) that interact with DNA via previously unidentified 32 and 110 kDa polypeptides. The 65 kDa activity was purified to homogeneity and consists of a homodimer of a 32 kDa polypeptide. In contrast to the other type I element binding factors, the 65 kDa activity partitions preferentially to the nuclear fraction during isolation. Levels of the 65 kDa activity increase dramatically in starved cells, raising the possibility that it might negatively regulate replication or transcription. By comparison, the other two binding activities were elevated slightly during macronuclear development, when the rDNA was undergoing DNA replication. Previous studies indicate that the initiation of rDNA replication is regulated by long range interactions between dispersed type I elements. Competitive DNA binding or cooperative protein-protein interactions between the factors described here may play a regulatory role in replication or expression of the rDNA minichromosome.